Signal environments that include noise and/or interference present a problem for communication systems that has been addressed using many techniques, including signal amplification, signal encoding, filtering by frequency range, and spread-spectrum modulation. Another technique that has been often used is that of antenna diversity. Referring to FIG. 1, in a communication system 100 that uses antenna diversity, at least two separate antennas 105, 110 are used as inputs to a single receiver system 115. For each data packet, the receiver 115 is designed to select which antenna input is providing the best signal reception for each data packet. Antenna diversity is most useful when operating in a wireless communication system, such as a system that uses radio communication techniques.
Antenna diversity techniques for signal reception in noisy environments and interference environments are very well known in the literature and are the subject of many patents. For example, see U.S. Pat. Nos. 5,077,753; 5,838,742; 6,061,574; 6,115,406; 6,215,812; 6,215,814; 6,229,842; 6,240,149; 6,256,340; 6,259,721; and 6,278,726, and European Patent Office Publication Number 0 642 243 A1, the contents of each of which are incorporated herein by reference.
Referring again to FIG. 1, one type of interference that is often encountered in communication systems is known as multipath interference. Multipath interference occurs when a signal propagates by more than one path between the transmitter 120 and the receiver 115. For example, a signal may propagate directly by line-of-sight along paths 125 between the transmitter 120 and the receiver 115, and it may also “bounce” off of the ground or some other reflective object en route along path 130 between the transmitter 120 and the receiver 115. When a signal arrives at a receiver after traveling by two or more paths, interference occurs because of the different path lengths, which causes the transmission time to differ and thus leads to the two or more separate reception of the signal to interfere with each other. Furthermore, even if the receiver is designed to filter out other signals based on frequency or coding or modulation, the receiver will typically be unable to distinguish between the multiple receptions of the same signal, because the received signal is exactly the signal for which the receiver is tuned and ready to receive.
Referring to FIGS. 2 and 3, in many communication systems that use antenna diversity to counteract the effects of noise and interference, signal-to-noise ratio (SNR) is the quantity that is typically measured and used as the determinant in the selection of which antenna should be employed. For example, SNR may be determined by measuring the amount of gain applied by a variable-gain amplifier (VGA) to keep the circuit input constant. A gain level 205 applied to noise may be compared with a gain level 210 applied after the start of a data packet, and the SNR is measured by taking the ratio between the two gain levels 205, 210. As another example, a signal power level 215 may be measured and compared with a noise power level 220, and the signal-to-noise ratio may be computed by dividing the signal power level 215 by the noise power level 220 (or, if the power levels are expressed in decibels, by subtracting the noise level 220 from the signal level 215).
FIG. 3 shows a block diagram for a conventional communication system 300 designed to select an antenna from two or more antennas in an antenna diversity scheme by using the signal-to-noise ratio as the determinant. The system 300 includes at least two receive antennas 305 that feed the received analog signal into a multiplexer 310. The signal passes through a low noise amplifier 315 before its frequency is converted to an intermediate frequency IF by a converter 320. The signal is then processed by a variable-gain amplifier 325, and then down converted to the baseband frequency by converter 330. The analog signal is digitized by an A/D converter 335, and then filtered by a filter 340 to remove signals outside its frequency range. Finally, the digital signal passes through an automatic gain controller 345, which effectively sets the signal level to a predetermined value and thereby measures the SNR. The predetermined value is set by using a control signal with a known amplitude as a reference, so that the signal power at the input of the automatic gain controller (AGC) 345 remains constant. The control signal is generated by the AGC 345, and then fed back to the variable-gain amplifier (VGA) 325 to adjust the gain level of the VGA 325. Based on the measured SNR, an antenna selection is made by unit 350.
However, in a multipath signal environment, the system performance may depend more upon characteristics related to the multipath nature of the environment than upon the SNR, especially because SNR is generally not a reliable indicator of multipath effects. Hence, to optimize system performance, multipath factors are taken into account. Accordingly, there is a need for a new method of selecting a receive antenna in an antenna diversity system when operating in a multipath signal environment.